<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        body {
            background-image: url(images/1.jpg);
        }
        
        li {
            list-style: none;
        }
        
        ul {
            margin: 100px auto;
            width: 800px;
        }
        
        ul li img {
            float: left;
            width: 200px;
        }
    </style>
</head>

<body>
    <ul>
        <li>
            <img src="images/1.jpg" alt="">
        </li>
        <li>
            <img src="images/2.jpg" alt="">

        </li>
        <li>
            <img src="images/3.jpg" alt="">

        </li>
        <li>
            <img src="images/4.jpg" alt="">

        </li>
    </ul>
    <script>
        var imgs = document.querySelector('ul').querySelectorAll('img');
        for (var i = 0; i < imgs.length; i++) {
            imgs[i].onclick = function() {
                document.body.style.backgroundImage = 'url(' +
                    this.src + ')'
                    // "url(' + this.src + ')"
            }
        }
    </script>
</body>

</html>